Improvement in quality and performance in the equine gene pool must begin with selection of only the very best individuals for breeding purposes. Because up to 90 percent of male horses are not of breeding stallion potential, gelding is very common. Due to a decrease in the production of androgens (the male hormones) after castration, geldings generally have a more stable disposition than stallions. Gelding makes a male horse suitable for a greater range of uses.
Male hormones are responsible for much more than the desire and capability to breed mares. Athletic performance can be helped or hindered by testosterone. It can make a stallion perform with more energy and brilliance than a gelding yet it can also serve to distract a stallion from the work at hand. Similarly, secondary sex characteristics, such as muscle bulk, that are influenced by testosterone production can manifest as desirable muscle definition and strength or as an undesirable cresty, thick, and inflexible neck.
Gelding is often used as a means to modify a horse's behavior. Vocalization, fractious behavior and sexual interest in mares are frequently the undesirable characteristics noted in the yearling and two-year-old stallion. Although sexual interest is desirable in a stud, sexual aggressiveness is inappropriate and dangerous in a performance animal. The urge to copulate is just one part of the breeding ritual; related breeding behaviors include forms of whinnying, squealing, pushing, rearing, striking, and biting - all socially acceptable behaviors among horses but not between people and horses. A small percentage of young male horses exhibit sexual frustration or self-mutilation tendencies. If such a horse is not intended for breeding, it is best to geld him before habits get established.
Although gelding will remove the underlying cause for such behaviors, it will not change poor manners and bad habits. This must be accomplished by proper training. Behavior learned before gelding, especially if it involved breeding, will not disappear instantly after castration, and perhaps will never be eliminated completely.
Some horses retain sexual behaviors after gelding and are often called "proud cut". In the past this was said to be due to some testicular tissue being missed during the gelding procedure allowing testosterone production (but not sperm production) to continue. In some cases, this may have been true, especially considering the variety of crude methods of castration practiced over the last 2000 years. However, today, with the availability of restraining drugs and the level of knowledge and surgical techniques, it is unlikely that missed testicular tissue is the cause for the estimated 25 percent of geldings that are said to exhibit some type of stallion behaviors. Since the adrenal glands (located near the kidneys) also produce testosterone, it is thought that the cause of so-called "proud cut" behavior may be due to the (hyper)activity of a particular horse's adrenal glands. Other stallion-like behaviors may simply be poor manners due to inadequate training.
WHEN TO GELD
The testicles of the normal male horse descend from the abdomen into the scrotum around birth. Gelding can be performed soon after birth but a delay is traditional for several reasons. First of all it gives foal owners more time to determine if a young horse is stallion potential. Secondly, it allows masculine characteristics, such as muscle definition, strength, and aggressiveness to develop.
When to geld is largely a management decision. Often, facilities require that young horses of both sexes be housed together, so gelding at about 12 months has become popular in order to avoid accidental breeding at puberty (18 to 24 months). It is best to assess each individual though, to determine the optimum gelding time. Some weanlings become preoccupied with nearby mares and may go through or over fences to get near them. In other cases, a long yearling may only quietly watch the mares. Others may develop an obsession with their penises and may devise various means of masturbation or self-mutilation. Other early gelding candidates include those that show premature signs of excess bulk such as a thick, cresty neck. Such individuals might best be gelded at eight months or earlier while others remain very supple and moderate in musculature well into their two-year-old year.
Therefore, depending on management and the tendencies of each individual, gelding usually takes place between six and twenty-four months of age.
Research has shown that there is little difference in the behavior change toward people in horses gelded before puberty (18-24 months) and those gelded after puberty. However, horses gelded after puberty tend to retain a greater amount of their former horse-to-horse behaviors - sexual drive, vocalization, and body language - than those gelded before puberty.
Early spring and late fall are the traditional seasons for gelding. Flies are usually not a serious problem for the healing wound site and the lower seasonal temperatures do not exaggerate swelling of the sheath. However, the mud characteristic of spring and fall may make dry, sanitary conditions more difficult to provide and maintain. the above was taken from an article by Cherry Hill
Farmers Almanac says :
Best days to Castrate animals - Jan.19, 20 and Feb. 15,16 http://www.almanac.com/astrology/index.php